I want it to be Ryuuki! IMO Seiran's and Shuurei's relationship resembles more of a protective older brother and little sister, he is even a bit of a mother hen with her really. So the idea of them ending up together makes me feel queasy.
The only other person that comes to mind is minister Kou, he is the prettiest after all I wish they did show his face in the anime

RYUUKI! would it be obvious people! gosh! for those who said seiran would end up with shuurei you are absolutely wrong..he is more than a big brother for shuurei and ryuuki and would always be as you said and duh?shuurei and seiran had been together since childhood and he never said that he likes her and always calls her "my princess" and obviously ryuuki and shuurei fell in love the first time they had met..well shuurei avoided her feelings for ryuuki at first but she followed him to ranshuu..and cried for him..did she cried like that for seiran? NO..if your watching the anime episodes in season 2..plus ryuuki would never married jyussen ran( shuei's sister) as he said "he loves shuurei" meaning he aint have no love for her..plus if your reading the manga (SPOILER ALERT!) ( ryuuki married shurrei and had they had a daughter) and if you watch closely in the ending of season 2 is ryuuki with that ran princess? NO! he is with shuurei..if you are asking if shuurei can be an empress I think yes, as long as she is with ryuuki she can surpass all the obstacles... she cried thinking she could not even marry him..would that be obvious she loves him? and I don't think that Ran princess or seiran would interfere..why? because the Ran princess knew shuurei and ryuuki love each other..as a woman would you be happy to be married like that?huh?as for seiran, If he has any feelings for shuurei he should had relinquished it..after all just what you had said he is ryuuki's brother, I know he'll wish them pure happiness and give way for his little bro whom he love so much.. even though ( SPOILER!) (shuurei died after giving birth to their child)(that's why I don't wanna have season 3) plus shuurei is the only one that can make ryuuki happy even though he is a king meaning there is no one befitting the title "empress" or " consort" than shuurei

and as for the one who said shuurei only love ryuuki 1% then how come she rushed to ranshuu and cried for ryuuki? gosh! only reading the manga but not analyzing the anime..plus in the end of season 2 shuurei doesn't say no to ryuuki right? meaning she gave him chance or give a chance to ryuuki's proposal to her right guys?